The present invention relates to a supply system for fuel cells of the solid polymer electrolyte type for hybrid vehicles.
It is known that technology in the transport field has been directed most recently towards the use of electric motors instead of internal combustion engines because of the high output of the former and because they have practically no impact on the environment.
In order to power the electric motors of transport vehicles it has been proposed to use fuel cells which are able to supply the average daily power required by the transport system, supplemented by the use of buffer batteries to provide the peak power required.
Of the fuel cells, the solid polymer electrolyte type, which operate at a temperature of the order of 70.degree.-120.degree. C., appear to be the most suitable for this type of use.
As is known, fuel cells make use of the exothermic oxidation reaction of hydrogen (or of gases containing hydrogen) with oxygen (generally oxygen in the air) with a conversion yield of the order of 40%.
It is thus clear that a fuel cell (or more properly a battery or stack of fuel cells) of such a dimension that it can produce electric power of the order of 60 KW which may be required by an urban transport vehicle such as a bus, and considering the losses due to incomplete combustion, develops a thermal horsepower of the order of 60-80 KW which must be dissipated to a great extent to ensure the necessary working conditions for the fuel cell.
Cooling is achieved by demineralised water which is circulated in the fuel cell and which also ensures the humidification of the catalytic membranes of the fuel cell, which is essential for them to function correctly.
The cooling water, partially consumed and dispersed in the humidification section by transport in the gases discharged from the cell, is supplemented by the recovery of some of the water produced in the cell by the oxidation reaction and must in turn be cooled by means of radiators so as to give up its heat to the ambient air.
To ensure that the fuel cell works efficiently in relation to its dimensions, it is necessary for it to operate at a pressure above atmospheric, and of the order of 3.5 bars absolute
This means that the cell must be supplied with hydrogen, air for combustion and cooling water at this pressure and at an optimum inlet temperature of the order of 70.degree. C.
The adiabatic compression of the air from the ambient temperature and pressure to the working conditions results in a temperature rise of the order of about 80.degree. C. degrees so that, in general, the compressed air must also be cooled.
It is thus necessary to provide heat exchangers which are able to contain a pressurised fluid (water or compressed air) and are at the same time efficient and, at least in the case of compressed air, to provide shut-off and bypass members for the exchanger which must not only operate under pressure but must also ensure, with minimum load losses, relatively high flow rates of the order of 20-25 litres per second of air at standard atmospheric pressure (ten litres per second at the operating pressure).
The apparatus thus becomes bulky and expensive and it is difficult to satisfy the requirements of compactness and safety which the plant must have.
A further problem is constituted by the fact that although from a theoretical point of view, all the hydrogen supplied may be used in the conversion reaction, in order to achieve an acceptable yield from fuel cells in terms of voltage and current supplied, it is necessary for twice the stoichiometric quantity of hydrogen to be supplied.
It is known that the excess hydrogen, partially exhausted, is discharged periodically to the exterior, with a not inconsiderable loss.